New Post: Using with GPL project?

I was going to say you can't, but there might be a workaround: You can use it as long as it is clear to developers that will use your open source project that the portion(s) from Extended WPF Toolkit are not licensed under GNU GPL, but rather they are MS-PL. So you can state that in the place(s) where licensing is concerned. Also, don't change the notice in any Extended WPF Toolkit source code files you include with the project.

The important thing is that you don't relicense portions of Extended WPF Toolkit to others under terms and conditions that give them more rights than what MS-PL does... The rights are different and incompatible.